There is a continuing effort to improve absorbent products, such as diapers, sanitary napkins, wound dressings, bandages, incontinent pads and the like that absorb body fluids, contain them well without leakage, and minimize skin wetness that can cause irritation. Generally, it is preferred that such absorbent products be soft and readily conform to body contours.
Many earlier absorbent products, in particular, diapers and sanitary napkins, contained an absorbent batt comprising tissue wadding or plies of paper tissue disposed between an impermeable backing sheet and a moisture permeable body facing sheet. The tissue wadding was not particularly desirable because it lacked a high absorptive capacity and imparted stiffness to the product.
Tissue wadding was replaced by a layer of individualized fluffed wood pulp fibers generally uniformly dispersed throughout the batt, such as that disclosed in U.S. Pat. No. 2,788,003. The fluffed wood pulp fibers provide a soft, high bulk, conformable product with an absorbent capacity and containment ability exceeding that of tissue wadding. However, although fluffed wood pulp fibers increase the fluid capacity of absorbent pads, the efficiency with which the capacity is used remains poor. This is because uniformly dispersed pulp does not effectively transport fluid away from areas of localized concentration. Body fluids are generally deposited in a localized area at or near the point of discharge and will follow a path of least resistance, which is usually the closest edge of the batt where fluid is no longer contained. This is particularly true when the absorbent product has an impermeable backing sheet because then the only path for fluid to escape is via the edges. Thus, the ineffective moisture conducting powers of a uniformly dispersed wood pulp batt contribute to the product's susceptibility to leaks.
The recognition that densified pulp is able to transport fluid effectively prompted the development of multilayered absorbent products wherein the various layers of the product are of different densities. See Burgeni and Kapur, "Capillary Sorption Equilibria in Fiber Masses," Test Res. J. 37(5):356 (1976) In manufacturing such a product, the various layers may be made separately, for example, by compression, and then combined to one structure with a capillary gradient. A traditional layered structure wherein a high density cotton core is wrapped with a loose, low density cotton fiber, is disclosed in U.S. Pat. No. 3,771,525. Another layered structure is shown in U.S. Pat. No. 3,017,304, which discloses an absorbent product containing a densified, paper-like layer. The paper-like layer acts as a wick so that liquid placed on this layer tends to move rapidly along the plane line of the layer. The paper-layer concept was used to improve the absorptive efficiency of the fluffed wood pulp fiber.
As well known to those of ordinary skill, the concept of combining a wicking layer with fluffed wood pulp fibers has gained wide acceptance in many absorbent products. For example, a diaper which incorporates this paper-like layer combined with fluffed wood pulp is disclosed in U.S. Pat. No. 3,612,055. This diaper construction is described as keeping moisture away from an infant's skin, while at the same time handling a full volume discharge of urine. The diaper is a multi-layer diaper comprising a fibrous facing layer in the form of a homogenous blend of short and long fibers which is brought into contact with the infant's skin, a layer of highly porous, loosely compacted cellulosic batt, a paper-like densified, highly compacted cellulosic fibrous layer integral with the loosely compacted batt, and an impervious backing sheet adhered to the densified layer throughout the interface with the densified layer.
A method of manufacturing a self-sustaining absorbent fabric is disclosed in U.S. Pat. No. 4,134,948. This method provides an absorbent fabric comprising an air laid, randomly arranged, intermingled cellulosic fibrous batt having a plurality of high loft, loosely compacted regions separated from each other by highly compressed regions. The compressed regions are formed by moistening the batt, embossing the batt for providing a pattern in the surface, and applying an adhesive material to the patterned surface. The adhesive penetrates through the compressed regions to form bonded fiber networks and partially penetrates through the high loft absorbent regions. The highly compressed regions have high capillary forces which aid in transmitting fluids along the fibrous structure, and the unbonded interiors of the high loft regions provide high capacity regions for storing such fluids.
Unfortunately, the fluffed wood pulp products discussed above do little to minimize potentially irritating skin wetness. U.S. Pat. No. 3,768,480 discloses an absorbent product wherein the facing layer has a wettability gradient that gradually increases within the facing layer to promote flow of fluid through the facing layer and into the batt, thereby providing a relative dry surface in contact with the skin. The wettability gradient is achieved by gradually decreasing the proportion of long fibers and increasing the level of short fibers in a blend, with the greatest concentration of long fibers being adjacent to the outer face of the facing layer, and with the concentration of short cellulosic fibers gradually increasing from the outer face to the inner face adjacent to the batt.
Even though the above-described products make much greater use of the capacity of the absorbent batt, none of them totally contain the absorbed liquid. It is probable that these products will leak before the full capacity of the batt is used for absorption. Although the recently introduced elastic leg or stretch diapers, such as those disclosed in U.S. Pat. Nos. 3,860,003, 4,050,462, and 4,324,245, improve containment of liquid, the elasticized portion itself can be irritating to the skin. Moreover, because the products fit more tightly, less air circulation is permitted which also enhances irritation.
The addition of superabsorbents into the storage zone of the product further improves the performance of the absorbent product. Superabsorbent materials are materials which will absorb many times their weight of liquid. U.S. Pat. No. 4,540,454, discloses a relatively thin absorbent product that utilizes superabsorbents. The product comprises a wicking layer and an absorptive layer superimposed upon one another, for example, by air layering. The absorptive layer is a low density, resilient, fibrous web consisting of randomly disposed, frictionally entangled fibers, resulting in a web having a dry bulk recovery of at least sixty percent, an initial dry bulk of at least 20 cc/gm and a weight less than about 2 oz/yd. The fibrous web is used to spatially distribute superabsorbent material so that upon exposure to an aqueous material, swelling occurs with minimal interference from adjacent superabsorbing material. The wicking layer is a high density transporting structure made of particles such as cellulosic fibers and/or peat moss. The layers are compressed at a pressure adequate to collapse the entire structure to provide intimate contact between the two layers and a structure that is one-half its original thickness.
It is well known that absorptive materials with low density have great liquid retaining capacity but poor liquid transmitting capacity, whereas absorptive materials with high density have great liquid transmitting capacity but poor liquid retaining capacity. It is also well known that flow and diffusion in capillary systems takes place in the direction from large capillaries (areas having absorptive materials with low density) to small capillaries (areas having absorptive materials with high density). In attempts to attain a balance between liquid retaining and liquid transmitting properties, and to maximize utilization of absorptive materials, prior art absorbent structures have included various layers of mutually differing densities.
U.S. Pat. No. 4,818,315, discloses a method of manufacturing a fibrous absorptive body having a continuous density gradient. In this method, absorptive fibers and a thermoplastic binding agent are formed into a homogenous intermixture in the form of a non-compressed web. The web is heated at a temperature above the melting point of a bonding fiber to activate the binding agent and thereby interconnect the absorptive fibers. The web is then cooled to a temperature below the binding temperature of the binding agent, and subsequently compressed between two rollers. One of the two rollers is at a temperature below the binding temperature of the binding agent and the other of the two rollers is at a temperature above the binding temperature of the binding agent. The desired effect is that binding decreases in a direction away from the opposite side of the web so as to produce a continuous density gradient in the web after its passage between the two rollers.
Recently, research has focused on providing a thin absorbent product that effectively transports fluid away from the body facing layer and utilizes its storage capacity efficiently. Despite the advent of many new thin absorbent products and methods for making the same, currently available products continue to exhibit problems such as leakage, staining, skin wetness and irritation. Accordingly, there is a need for improved absorbent structures that effectively transport fluid away from the body facing layer and utilize storage capacity efficiently.